linux-kernel - Re: Time slice for SCHED

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Message-Id: <1234357350.23438.149.camel@twins>
Date:	Wed, 11 Feb 2009 14:02:30 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	J K Rai <jk.anurag@...oo.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: Time slice for SCHED_BATCH ( CFS)

On Wed, 2009-02-11 at 17:58 +0530, J K Rai wrote:
> Can we say that given n cpus and m processes the time-slice will
> remain constant under SCHED_BATCH or so? 

Only if those processes remain running, if they get blocked for whatever
reason it'll change.

> Can we form some kind of relationship? 

Sure,

latency := 20ms * (1 + log2(nr_cpus))
min_granularity := 4ms * (1 + log2(nr_cpus))
nr_latency := floor(latency / min_granularity)


           latency ; nr_running <= nr_latency
period = {
           nr_running * min_granularity ; nr_running > nr_latency


slice = task_weight * period / runqueue_weight


as you can see, its a function of the number of cpus, as well as all
other running tasks on a particular cpu.

Load-balancing of course makes this an even more interesting thing.


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